The Ultraluminous M81 X‐9 Source: 20 Years’ Variability and Spectral States

The source X-9 was discovered with the {\it Einstein Observatory} in thefield of M81, and is located in the dwarf galaxy Holmberg IX. X-9 has a 0.2-4.0keV luminosity of $\sim 5.5\times 10^{39}$ ergs~s$^{-1}$, if it is at the samedistance as Holmberg IX (3.4 Mpc). This luminosity is above the Eddingtonluminosity of a 1~$M_{\odot}$ compact accreting object. Past hypotheses on thenature of this Super-Eddington source included a SNR or supershell, anaccreting compact object and a background QSO. To shed light on the nature ofthis source, we have obtained and analyzed archival data, including the {\itEinstein} data, 23 ROSAT observations, Beppo-SAX and ASCA pointings. Ouranalysis reveals that most of the emission of X-9 arises from a point-likehighly-variable source, and that lower luminosity extended emission may beassociated with it. The spectrum of this source changes between low and highintensity states, in a way reminiscent of the spectra of galactic Black Holecandidates. Our result strongly suggest that X-9 is not a background QSO, but abonafide `Super-Eddington' source in Ho IX, a dwarf companion of M81.Comment: 31 pages, 10 figures, ApJ - accepted for publicatio